With snow blanketing much of the East Coast, it’s not surprising that one of the most abundant species now visiting seed feeders in my Washington, DC, Certified Wildlife Habitat® is the dark-eyed junco. Nicknamed “snowbirds,” dark-eyed juncos are themost common backyard feeder birds across much of North America during winter, according to the Cornell Lab of Ornithology.

Among the most variable of all North American birds, the dark-eyed junco features several distinct color forms, five of which were once considered separate species. Today six groups of subspecies—or types—of a single species, Junco hyemalis, are accepted by ornithologists. The two most widespread are the “slate-colored junco” of the eastern United States and most of Canada and the “Oregon junco,” which inhabits much of the West.

Dark-eyed juncos breed and nest in coniferous or mixed-coniferous forests in Canada, the western United States and parts of the Appalachians during summer. Come winter, they spread out across much of the rest of the continent, then vanish again in spring.

Over the past several years, scientists have made some interesting discoveries about the social and breeding behavior of dark-eyed juncos. During winter, the birds form rigid hierarchies led by dominant males and their seemingly closest adult male buddies. Younger males and females often are pushed to the periphery or forced to migrate farther south to avoid confrontations with these “good old boy” networks.

Newer research reveals that another set of dynamics guides the birds when they pair up on their breeding grounds in spring. Scientists have found that males with higher levels of testosterone—no matter what their age—are irresistible mates for older, more experienced females. They also produce more offspring.

But there’s a catch: The scientists also discovered that these testosterone-pumped studs are not particularly good fathers, and their offspring are born smaller and die at higher rates. The hormonally charged birds are so eager to show off and chase after new partners that they don’t visit or bring food to their nests as often as their less popular rivals.

In addition, the über-juncos tend to die at a younger age. Not only does elevated testosterone make them less wary of predators, it also increases their stress levels and promotes production of corticosterone, a hormone that mobilizes quick-release energy but also breaks down protein and leads to the atrophy of muscles, feathers and organs.

There is only so much time and energy available in a given day, and evolution requires making some tough choices, notes Indiana University behavioral ecologist Ellen Ketterson, lead scientist of the research team. “If you’re a male junco,” she asks, “do you maximize the time you spend chasing partners or do you maximize the time you spend parenting?”

Backyard Tips: During winter, dark-eyed juncos are gregarious ground foragers that readily mingle with other birds in search of food. To attract them: Place food on the ground or in a platform or hopper feeder.

The birds’ favored fare includes hulled sunflowers, millet and finely cracked corn. Because their bills are relatively small, juncos are not able to handle larger seeds or sunflower seeds in the shell.

Voice: Song of all types is a rich trill on one pitch, like a chipping sparrow but not so dry. Call is a short, flat thtp, similar to a hummingbird call.